1. Field of the Invention
The present invention relates to an injection molding method and injection molding apparatus in which a resin molded article is obtained by injection molding a resin material.
2. Related Art
For example, in a resin molded article manufacturing method in Japanese Patent No. 3,611,058, in molding a molded article having a partial thick wall on a rear surface side, it is designed to prevent a shrink from occurring on a front surface side of the molded article.
That is, in Japanese Patent No. 3,611,058, in a cavity of a forming mold for molding the molded article, a pressurized fluid inlet is formed on the same side as a rear surface side on which the thick wall is molded. Also, on the rear surface side of the cavity, a portion corresponding to a base of the thick wall is formed using a heat insulating material.
Then, a cooling of a resin in the proximity of the base of the thick wall is more delayed by the heat insulating material than a cooling of a resin on the periphery of the resin, and the base of the thick wall and the proximity of the base are pressed against the front surface side of the cavity from the rear surface side of the cavity by a pressurized fluid which has been injected from the inlet into an area between the rear surface side of the molded article and the heat insulating material, thus preventing a shrink from occurring on the front surface side of the molded article.
However, in the existing injection molding technique, particularly, when molding a large-size resin molded article or a resin molded article having a length which is larger than a thickness or a width, in the event that a plurality of the pressurized fluid inlets (gas inlets) is provided along a passage direction of the resin in the cavity, it is not designed to prevent a shrink and the like from occurring on the front surface side of the molded article.
That is, when carrying out an injection molding of the large-size or long resin molded article, a certain amount of time (a few seconds) is required to cause a molten resin to reach a distal end of the cavity from a resin inlet (a resin inlet gate) in the cavity. At this time, in the event that, in the same way as in the existing technique, an introduction of a gas into the cavity from the inlet is carried out after the molten resin has reached the distal end of the cavity, a surface skin (a cured portion) formed on a surface of the molten resin which makes contact with the mold tool has grown considerably in the proximity of the resin inlet, leading to a reduction in effectiveness in preventing the shrink and the like.
Further, there is a case that a boss of circular cross-section is projected from a rear surface of a plate-like body in a resin molded article used as various component parts of an automobile in order to attach the resin molded article to another component member, or for strengthening purposes.
As a method for manufacturing a resin molded article including such a boss, there is, for example, one disclosed in JP-A-10-86169 and JP-A-2005-28731.
JP-A-10-86169 discloses that, in injection molding a resin molded article, when a molten resin to be formed into the resin molded article is cooling and curing, a compressed gas is supplied to an inside of a boss, and a portion of the molten resin near a base of the boss is pressed against a mold tool by the compressed gas. Also, JP-A-2005-28731 discloses that, in manufacturing a resin molded article provided with a boss, a tubular pin and an inner pin are disposed in an inner portion of a cavity forming the boss, and, a compressed gas is supplied from a gap between the tubular pin and the inner pin, thereby pressing a molten resin which has filled an inside of a cavity.
Then, in JP-A-10-86169 and JP-A-2005-28731, a state of strain such as a shrink (a dimple formed on a surface) is prevented from occurring on a front surface of a plate-like body opposite a rear surface on which the boss stands.
However, even the existing resin molded article manufacturing method does not suffice for effectively preventing a state of strain such as a shrink from occurring on a front surface of the resin molded article having the boss.
That is, in the existing resin molded article manufacturing method, only an inner bottom of the boss is pressed by the compressed gas, so it is possible to prevent a shrink or the like from occurring on the front surface corresponding to an inner portion of the boss, but the manufacturing method does not suffice for preventing the shrink or the like from occurring on the front surface corresponding to an outer portion of the boss. Also, when causing the compressed gas to press only the inner bottom of the boss, in some situations, there is a possibility that the compressed gas enters an inside of the molten resin, and forms a hollow portion inside the resin molded article.